In general, to cool a plurality of cylinder liners in a water-cooling type diesel engine, a water jacket is formed in the region in the vicinity of each cylinder liner in a cylinder block so as to allow a coolant to be pumped to the water jacket.
As the cylinder liners are conventionally cooled, distribution of the temperature on the wall surface of each cylinder liner generally varies as represented by a curve A in FIG. 2.
With respect to the configuration of a water jacket for a comparatively small-sized diesel engine having a piston displacement smaller than five liters, a sectional configuration of the water jacket is dimensioned to have a narrower width W approaching the upper part thereof, i.e., the cylinder head side, as shown in a sectional view in FIG. 5. It should be noted that formation of the sectional configuration of the water jacket having a narrower width W approaching the upper part thereof in the above-described manner has been hitherto disclosed in an Official Gazette of e.g., Japanese Laid-Open Utility Model No. 153843/1985.
To assure that an engine generates a sufficiently large output with a supercharged intake air with the aid of a supercharger or the like, a proposal has been already made such that each cylinder liner is molded of a ceramic material or the like material so as to thermally insulate the whole cylinder liner. With respect to the engine constructed in the above-described manner, the temperature on the wall surface of each cylinder liner is distributed as represented by curve B in FIG. 2. As in apparent from curve B, the wall temperature is elevated not only at the upper part of the cylinder liner but also in the region extending from the central part toward the lower part of the cylinder liner.
The relationship between the temperature on the wall surface of each cylinder liner and the quantity of consumption of a lubricant oil is generally represented by the graph in FIG. 4. It has been found that the quantity of consumption of the lubricant oil is increased in substantial proportion to elevation of the temperature on the wall surface of each cylinder. For this reason, with respect to the afore-mentioned engine adapted to generate a large output with a supercharged intake air with the aid of a supercharger or the like, when the whole cylinder liner is thermally insulated, there arises the problem that the quantity of consumption of a lubricant oil increases because of the elevated temperature of the whole cylinder liner.
In addition, since the intake air is increasingly heated and expanded as the temperature of the whole cylinder liner is elevated, there arises other problems: intake air charging efficiency is degraded, properties in respect of the color of exhaust gas and the quality of particulates are deteriorated; moreover, the quantity of nitrogen oxides (NO.sub.x) increases due to elevation of the combustion temperature associated with elevation of the temperature on the wall surface of each cylinder liner at the end of a compression stroke.
On the other hand, as schematically illustrated in FIG. 6, a cooling system for a small-sized engine having a piston displacement smaller than five liters is constructed such that a coolant delivered from a water pump P is supplied to a water jacket c formed around a fore cylinder liner b, the coolant is then supplied to an intermediate cylinder liner b from the fore cylinder liner b and the coolant is finally supplied to a rear cylinder liner b from the intermediate cylinder liner b. It should be noted that among outlet ports on a cylinder block d which each communicate with a cylinder head (not shown) a rearmost outlet port e has a cross-sectional flow passage area twice that of other outlet ports e.
However, with respect to the cooling system shown in FIG. 6, since the fore cylinder liner b is sufficiently cooled by the coolant but the intermediate cylinder liner b and the rear cylinder liner b are insufficiently cooled by the warm coolant with an elevated temperature the temperature on the wall surface of each of the cylinder liners b located behind the fore cylinder liner b is elevated undesirably. For this reason, the conventional cooling system can not be employed especially for a large-sized engine adapted to generate a large output.
Additionally, where the water jacket c has a width W which is narrowed approaching the upper part thereof as shown in FIG. 5, and the cooling system is constructed such that a coolant flows from the fore side toward the rear side of an engine like the cooling system shown in FIG. 6, then the coolant flows at a lower speed in the region where the water jacket c has a narrower width, resulting in the cooling efficiency being degraded.
The present invention has been made with the foregoing background in mind and its object resides in providing a method of cooling a plurality of cylinder liners in an engine wherein the cooling efficiency of the cylinder liners is improved, the intake air charging efficiency is improved, properties in respect of the color of exhaust gas and the quality of particulates are improved and moreover the quantity of nitrogen oxides (NO.sub.x) in the exhaust gas is reduced substantially.